1. Field of the Invention
The present invention relates to a torque sensing device for use in a belt-driven conical-pulley transmission.
2. Description of the Related Art
Belt-driven conical-pulley transmissions, such as are employed for example in motor vehicles, generally include two pairs of conical disks that are encircled by an endless torque-transmitting means, for example a plate-link chain. By changing the spacing between the conical disks of each conical disk pair in opposite directions, the transmission ratio of the transmission can be varied continuously.
Advantageously, a conical disk pair, preferably the one on the power input side, includes an integrated torque sensor with which the torque acting from a drive engine is detected and a pressure between the conical disks of the corresponding disk pair is changed in accordance with the torque.
Such belt-driven conical-pulley transmissions with integrated torque sensor are described for example in published German patent applications DE 42 34 294 A1, DE 19 54 644 A1, DE 40 26 683, DE 195 45 492 A1 and DE 199 51 950 A1.
FIG. 4 shows a cross section through an input-side part of a belt-driven conical-pulley transmission. On an input shaft 10, which is made in a single piece with a fixed disk (not shown), a movable disk 14 is positioned so that it can be shifted axially but is non-rotatably connected to the input shaft.
On the back side of movable disk 14 in its radially outer area, a cylindrical ring 16 having two axially-extending annular walls spaced at a radial distance from each other is rigidly attached. A piston 18 operates within cylindrical ring 16 so that on the right side of piston 18, as viewed in FIG. 4, a first pressure chamber 20 is formed that can be subjected to hydraulic pressure through radial bores 22 in movable disk 14. An annular chamber 24 is provided between movable disk 14 and shaft 10, and a radial bore 26 and an axial bore 28 in shaft 3 allows changeable hydraulic pressure to adjust the transmission ratio.
Piston 18, which is of annular form, is rigidly connected to a support ring 30 which is substantially cup-shaped and is rigidly connected to input shaft 10. Ramp surfaces 32 are formed on the end face of support ring 30 that faces movable disk 14.
Also situated inside support ring 30 is an axially movable sensing piston 36 that is of annular form. Piston 36 carries a sealing ring that engages the outer surface of input shaft 10 and a further sealing ring that engages an inner circumferential surface of support ring 30. Sensing piston 36 includes an annular extension directed toward movable disk 14 and on which ramp surfaces 38 are formed that constitute countersurfaces to the ramp surfaces 32. Between ramp surfaces 32 and 38 are rolling elements, in the illustrated example balls 40.
Between sensing piston 36 and movable disk 14 a second pressure chamber 42 is provided, which is subjected to hydraulic pressure through a supply line 44 leading through the shaft, the hydraulic fluid being removable through a drain line 46 that is also formed in input shaft 3.
The effective cross section of the supply orifice 48 that leads into the second pressure chamber 42 is determined by the axial position of movable disk 14 relative to shaft 10. The effective cross section of the drain orifice 50 leading out of the second pressure chamber 42 is determined by the position of the sensing piston 36. The sensing piston 36 includes circumferentially-spaced axial arms 52 that extend through openings in the wall of support ring wall 30. The radial outer surfaces of the arms 52 are provided with axially and radially directed teeth, which mesh with inner teeth of an input wheel 54. Input wheel 54 is rotatably carried on input shaft 10 supported by a bearing 58 so that it is essentially axially immovable on the input shaft.
Radially within cylindrical ring 16, which is rigidly connected to movable disk 14, an annular ring component 56 is rigidly connected. An inner guide surface 58 is provided on ring component 56, against which the balls 40 lie, and which limits the radial outward movement of the balls.
The construction and the function of the arrangement described so far are known and will therefore be explained only briefly.
When there is a torque from the rotationally drivable input wheel 54 acting on sensing piston 36, that torque is transmitted via the ramp surfaces 38, the balls 40, and the ramp surfaces 32 to the support ring 30 and thus to the shaft 10. The ramp surfaces are designed in such a way that as the input torque increases the sensing piston 36 moves to the right, as viewed in FIG. 4, so that the drain orifice 50 is increasingly closed. As the effective size of the drain orifice 50 becomes smaller, the pressure in the second pressure chamber 42 increases, so that a pressure that is a function of the input torque acts against movable disk 14. With movable disk 14 shifted as far as possible to the left (maximum underdrive of the transmission), supply line opening 48 is closed, so that second pressure chamber 42 is unpressurized. Because the effective radius of guide surface 58 becomes smaller when movable disk 14 is repositioned to the right, as viewed in FIG. 4, the contact between the balls and the ramp surfaces is shifted in the radially inward direction as movable disk 14 is shifted increasingly to the right, causing the pressure in second pressure chamber 42 to be modulated increasingly depending upon the transmission ratio, since the slopes of the ramp surfaces depend upon the radial distance of the contact points between the balls and the ramp surfaces from the axis of input shaft 10. Thus, the slopes of the wedge surfaces are generally greater in the radially inner direction than in the radially outer direction.
The known torque sensing device, formed in particular by sensing piston 36, ramp surfaces 32 and 38, as well as guide surface 58 and balls 40, has the following particular characteristics:
As the speed of rotation of input shaft 10 increases, an increasing centrifugal force acts on the balls 40, which force is generally not completely absorbed by the reaction forces of the ramp surfaces and the guide surface, so that the pressure in second pressure chamber 42 become dependent upon the speed of rotation. That can cause slipping of the belt-driven conical-pulley transmission, or overloading, or even hysteresis.
When the slope of the ramp surfaces in the circumferential direction is dependent only on their distance from the axis of input shaft 10 (radial dependency), the perpendicular to the rolling surface on which the balls roll on the ramp surfaces has a radial component. The center point of the ball and the contact point between ball and ramp are therefore on different radii. This difference is approximately proportional to the axial displacement of the guide piston. As a result, the relationship between the contact pressure (pressure in the second pressure chamber) and the torque is dependent not only on the transmission ratio, but also upon the volume flow and the backup pressure of the hydraulic medium flowing through the second pressure chamber 42, and on the torque.
An object of the present invention is to provide a torque sensor with which the previously described unfavorable dependencies of the ascertained torque on the speed of rotation, the volume flow, the back pressure, and other variables are avoided.